1. Field of the Invention
This invention relates to a method for measuring the attenuation in light waveguides for communications transmission purposes and for coupling the test piece to the transmitter and receiver of the measuring apparatus.
2. Discussion of the Prior Art
To measure attenuation in optical light waveguides, only those modes must be picked up which can propagate in the light waveguide core. At discontinuities, such as splices, plug connections or branching points, leakage waves are excited which must be suppressed in the attenuation measurement. In addition, the light carried in the light waveguide jacket must be suppressed because it would stimulate too low an attenuation value.
The optical measuring signal is therefore conducted between the transmitter and the receiver via so-called mode strippers and mode mixers when entering as well as when leaving the test piece. These can be light waveguides, for instance, 500 m long and wound with a small radius, the geometry and index of refraction profile of which is adequate for those of the test piece.
Prior to the measurement, the test piece must be coupled into the ray path between the transmitter and the receiver. In order to do this with a minimum of losses, the light exit and entrance surfaces must be adjusted exactly. To do this is difficult because the lines are not terminated with self-adjusting plug devices. The light waveguides to be tested have a small numerical aperture and very small entrance and exit surfaces (about 0.003 mm.sup.2). For this reason, the adjustment, which is performed with three dimensional micro-manipulators, is almost impossible and at best is very difficult and time consuming.
Special devices for this purpose which solve the problem in a suitable form, are not available. Heretofore, as a laboratory makeshift, a light waveguide, serving as a mode stripper, has been inserted into the optical ray path, using a nondetachable splice to the test piece. However, this is very time consuming and therefore, uneconomical. It is therefore desirable to use mode strippers which are arranged in the vicinity of the transmitter and the receiver.
It is an object of the present invention to substantially simplify the difficult coupling operations described above.